Presently IEEE 802.3 local area networks, also known as Ethernets, utilizing unshielded twisted pair (UTP) wires or coaxial transmission lines, are limited by specifications to a maximum transmission frequency of 10 megahertz (MHz). This corresponds to a bit rate of 10 megabits per second (MBPS). Because existing UTP cabling for IEEE 802.3 has a specified maximum bandwidth of 10 MHz and the IEEE 802.3 encoding provides a corresponding bit rate of only 10 MBPS, if one desires to double the bit rate for the existing bandwidth and transmission media, then a new method of encoding data is necessary.
FIG. 1 shows the existing method of UTP operation using an IEEE 802.3 10-BASE-T configuration. Data is converted from parallel digital data 120 into serialized non-return to zero (NRZ) digital data 121 in the Serializer 101, also known as Media Access Controller, of Station 113. A station or node may be anything connected to a LAN, such as a computer, server, router, repeater, terminal or other peripheral device (printers, modems, or fax machines). Serialized NRZ data 121 is sent to the Manchester Encoder 102 (ME). ME 102 encodes clock and data together by manipulating the phase of the clock, thus generating another serial stream of encoded data 122. Manchester is the code or encoding method selected by the IEEE 802.3 specification by which clock and data are combined to provide a self clocked serial transmission. Encoded data 122 is buffered by transformer driver 103. Transformer driver 103 drives the coupling transformer 123 such that data propagates onto the UTP wires. The encoded data is received at regenerative repeater port 114 which comprises the receiving end of this communication channel. Encoded signals are received by the receiver transformer 129 and amplified by the receiver amplifier 107 to generate encoded data. Manchester Decoder 108 (MD) recovers clock and decoded data 131 from the encoded data. The MD performs Manchester decoding which is the reverse operation of Manchester encoding. Deserializer 109 converts the decoded data 131 from serial into parallel digital information 132.
Encoding methods or codes other than Manchester have been and can be used in serial communication systems. These include Biphase-Level (also known as Biphase-L or Inverted Manchester), FM0 (also known as Biphase-Space or Biphase-S), FM1 (also known as Split-Phase-Mark or Biphase-Mark or Biphase-M), Differential Biphase-Level (also known as Differential Biphase-L), and Differential Manchester. Because of a close relationship between these codes, it is easy to confuse the definition one with that of another. For this discussion we will consider Biphase-Level to be the inverted waveform of Manchester and Differential Manchester to be the inverted waveform of Differential Biphase-Level. Definitions for some of the above encoding schemes are found in Zilog 1982/1983 Data Book, Page 180 or Zilog Z16C30 Preliminary Product Specification, May 1989, Page 8, as well as the discussion below. Similar to Manchester encoding, each of these prior-art single encoding methods provide a bit rate of only 10 MBPS for a given maximum serial signal bandwidth of 10 MHz. Thus, if one desires to double the bit rate for an existing bandwidth and transmission media, then a new method of encoding data is necessary.
Another problem that arises in local area networks using coupling transformers is that over a period of time a DC offset can accumulate on the transmission line. DC offset can be caused on the LAN transmission line by an unequal number of time periods of positive pulses as compared with negative pulses within the serial signal transmitted. If DC offset accumulates beyond a tolerable limit, then it can cause the receiving amplifier to operate outside of its specifications, which in turn can cause errors in the received data.
In response to this problem, the tape recording industry developed the Miller Squared Code (Jerry Miller, U.S. Pat. No. 4,027,335). This is an attempt to generate a DC free code, also referred to as a DC balanced code, that improves upon a Miller Code (Armin Miller, U.S. Pat. No. 3,108,261) by removing a transition pattern within the Miller Code for a particular NRZ bit pattern. The Miller Code is often referred to as a (MFM) modified frequency modulation code or delay modulation code which is in a category of (RLL) run length limited codes.
Manchester Encoding is immune to this problem due to the fact that over time the number of transmitted negative and positive pulses is equal, which cancels out the DC offset. However, Manchester encoding is limited to a maximum bit rate of 10 MBPS by IEEE specifications and cabling. New encoding methods, created to improve LAN system performance, must also solve the DC offset problem.